Too rough to be a Hypsibius: multipopulation survey accentuates the phylogenetic position and apomorphies of Hypsibius scabropygus (Eutardigrada: Hypsibiidae)

Abstract

Tardigrade taxonomy is undergoing a turbulent phase of reclassifying various species that have previously been gathered within large wastebasket genera, such as Diphascon or Hypsibius. The application of genetic sequencing, paired with detailed analysis of fine morphological features in 3D using scanning electron microscopy, allow for revealing new phenotypic characters used in systematics. At the same time, resampling of previously well-sampled regions, such as the Western Palaearctic, delivers fresh and precious data. In this paper, the second from a series of publications integratively addressing the tardigrade fauna of Denmark, we focus on Hypsibius scabropygus and related species, which appeared to be common and abundant in cryptogam samples. As previously suggested by scarce molecular and morphological evidence, Hypsibius scabropygus is considerably different from other representatives of the genus Hypsibius by the presence of ramazzottiid characters. These include Ramazzottius-like claws with thinned, and thus more flexible, primary branches of external/posterior claws, apophyses for the insertion of the stylet muscles asymmetrical with respect to the frontal plane, rudimentary elliptical sensory organs, buccal tube slightly bent in posterior portion, and, finally, eggs with distinct chorion granulation laid in exuviae (known for some hypsibiids and acutuncids), and not smooth, which is otherwise typical for most hypsibiids. Consequently, the group of Hypsibius species with two small granular macroplacoids in the pharynx and pronounced cuticular sculpturing in the form of dorsoposterior protuberances or whole-dorsal tubercles is erected as Parahypsibius gen. nov. New DNA barcodes strongly support the monophyly of Hypsibiidae and recent changes in the classification of Pilatobiinae. We argue that some still poorly resolved nodes in the hypsibiid tree are the result of incomplete representation of particularly uncommon evolutionary lineages. Our abundant cryptogam samples from across Denmark demonstrate the value of involving local communities and utilizing citizen science in biodiversity monitoring and documentation.